In order to achieve reasonable energy economy most industrial evaporations are conducted with multiple effects (thermal drive) or with a steam compressor (electrical/mechanical drive). An example of multiple effect evaporation is disclosed in U.S. Pat. No. 3,884,767. Compression is disclosed in U.S. Pat. No. 3,234,109. Mechanical vapor recompression (MVR) usually incorporates only one stage of evaporation, thus keeping the required temperature lift and compression ratio very low. This results in a high Coefficient of Performance (COP), i.e., large quantity of water evaporate per unit of electrical input. Thermal evaporators achieve large COP with multiple effects; as a first approximation, the mass of water evaporated divided by the mass of driving steam supplied is equal to the number of effects.
A problem shared by both approaches to evaporation is that many feed materials are temperature sensitive, and must be limited to approximately 100.degree. C. or lower. For example, seawater experiences excessive scaling at high temperature, and stillage contains proteinaceous material which breaks down under heat. The practical result is that both technologies operate either near or well below ambient temperature. With MVR, this requires a large and costly compressor. With multiple effects, all of the effects must be designed within the relatively narrow temperature range between 100.degree. C. and ambient, imposing low temperature differences (large surface areas) and also requiring the lower temperature effects to operate deep in the vacuum region. Another disadvantage is that when the thermal heat supply is at high temperature, the high temperature availabilities cannot be used.
It has been disclosed in the prior art to apply absorption heat pumping to the evaporation process. In conventional closed-cycle absorption heat pumping, a COP (Coefficient of Performance) of about 1.7 is obtained from a single generator cycle, and 2.3 with a two-stage generator. Thus, the overall COP of the combination of evaporator plus absorption heat pump is the product of the two individual COPs. For example, a three-effect evaporator (COP.apprxeq.3 ) combined with a single stage AHP (COP.apprxeq.1.7) would be expected to have a COP of 5.1.
Example disclosures of applying absorption heat pumping to evaporation include U.S. Pat. Nos. 3,203,875, 3,261,766, 3,288,686, 3,692,634, 4,209,364, 4,350,571, and 4,379,734.
Both mechanical vapor recompression and absorptive vapor recompression are preferably done in an open cycle mode, to avoid the energy and capital expense of an extra heat exchanger temperature difference .DELTA.T.
The problem with absorption heat pumped evaporation, which is not known to ever have been successfully applied, is that three more sizable heat exchangers are incorporated (absorber, desorber, and solution heat exchanger), and yet the COP is only increased by a factor of 1.7. Thus it is still necessary to have multiple evaporation effects, with the problems and expense they entail.
What would be desirable, and one objective of this invention, is an absorption cycle steam recompressor having a COP of at least 2.5, and preferably much higher, such that when it is applied to evaporation an acceptable overall COP can be obtained with only one stage of evaporation. The single evaporation stage can be at close to the highest allowable temperature, which keeps the pressure up and the cost down. If the three or four heat exchangers of the absorption cycle achieve a COP of 6, that is equivalent to an electric compression evaporator with a COP of 18, due to the higher cost of electricity and the thermal-to-electric conversion efficiency. The high efficiency absorption cycle evaporator would be as simple and reliable as a multi-effect evaporator, and less costly than either type of conventional technology at comparative efficiency level.
One absorption cycle known in the prior art to have a cycle COP greater than that of a single stage cycle is the "Generator-Absorber Heat Exchange" (GAX) cycle. A closed GAX cycle using the absorption working pair NH.sub.3 -H.sub.2 O has been disclosed for residential and commercial heating applications by B.A. Phillips and also by R.A. Modahl and F.C. Hayes in the August 1985 Proceedings of the DOE/ORNL Heat Pump Conference-CONF-841231, Oak Ridge, Tennessee. In that cycle, a significant fraction of the generation heating is supplied by the hottest section of the absorption step. Thus only the remaining generation duty need be supplied by external heat. "In the heat pump art", "closed cycle" indicates that a separate hermetically sealed fluid is used in the heat pump, whereas "open cycle" indicates that some fluid from the process being heat pumped is incorporated directly into the heat pump.